From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Brian Geffon <bgeffon@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"Michael S . Tsirkin" <mst@redhat.com>,
Arnd Bergmann <arnd@arndb.de>,
LKML <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
Linux API <linux-api@vger.kernel.org>,
Andy Lutomirski <luto@amacapital.net>,
Will Deacon <will@kernel.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Sonny Rao <sonnyrao@google.com>, Minchan Kim <minchan@kernel.org>,
Joel Fernandes <joel@joelfernandes.org>,
Yu Zhao <yuzhao@google.com>, Jesse Barnes <jsbarnes@google.com>,
Florian Weimer <fweimer@redhat.com>
Subject: Re: [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap().
Date: Fri, 21 Feb 2020 15:23:47 +0300 [thread overview]
Message-ID: <20200221122347.vlw6r7felchtcxgq@box> (raw)
In-Reply-To: <CADyq12y2uJh2ROBajA1D5H6OLEyFrKKEXH=yQC-8ELF0+UiNzw@mail.gmail.com>
On Thu, Feb 20, 2020 at 03:55:53PM -0800, Brian Geffon wrote:
> Hi Kirill,
>
> > I have hard time understanding the case when new_len != old_len.
> >
> > Correct me if I'm wrong, but looks like that you change the size of old
> > mapping to be the new_len and then create a new of the same new_len.
> >
> > This doesn't look right to me.
> >
> > In my opinion, MREMAP_DONTUNMAP has to leave the old mapping intact. And
> > create the new mapping adjusted to the new_len.
> >
> > Other option is to force new_len == old_len if MREMAP_DONTUNMAP is
> > specified. It would simplify the implementation. And I don't see why
> > anybody would really want anything else.
>
> I had been approaching this as, "do what mremap would have done in
> this situation except skip the last step." Meaning, whatever the final
> state of the old mapping was MREMAP_DONTUNMAP meant that you should
> just not do the unmap operation on the old mapping at the end. But I
> understand why it's confusing, especially when in the case of the VMA
> growing you're left with the old vma of size old_len and the new_vma
> of size new_len but only containing old_len worth of pages.
> Personally, I don't think this is a problem having that behavior
> because it can be documented and it just adds a small amount of
> flexibility.
>
> Nonetheless, I agree with you and I also cannot come up with a
> situation where you'd actually want to do this so I'm willing to
> restrict it to old_len == new_len and return -EINVAL if not, it
> simplifies it a bit and accounting becomes a easier because the
> outcome is always the same two mappings of size old_len and the size
> of the locked_vm never changes. We can always allow the resize
> operation later if there becomes a need. If everyone is okay with this
> restriction I can send a new patch.
If anyone would want to chagne size it can be archived by followup
mremap() operations. There's no need in one-shot operation.
--
Kirill A. Shutemov
next prev parent reply other threads:[~2020-02-21 12:23 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-18 17:32 [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap() Brian Geffon
2020-02-18 17:32 ` [PATCH v6 2/2] selftest: Add MREMAP_DONTUNMAP selftest Brian Geffon
2020-02-19 20:39 ` [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap() Andrew Morton
2020-02-19 21:38 ` Lokesh Gidra
2020-02-19 23:23 ` Minchan Kim
2020-02-20 11:57 ` Kirill A. Shutemov
2020-02-20 23:55 ` Brian Geffon
2020-02-21 12:23 ` Kirill A. Shutemov [this message]
2020-02-20 17:15 ` Minchan Kim
2020-02-20 18:36 ` Brian Geffon
2020-02-20 18:45 ` Brian Geffon
2020-02-20 19:14 ` Minchan Kim
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